The present invention relates generally to the field of germanium compounds. In particular, the present invention relates to the certain germanium compounds suitable for use in vapor deposition processes.
Metal films may be deposited on surfaces, such as non-conductive surfaces, by a variety of means such as chemical vapor deposition (“CVD”), physical vapor deposition (“PVD”), and other epitaxial techniques such as liquid phase epitaxy (“LPE”), molecular beam epitaxy (“MBE”), chemical beam epitaxy (“CBE”) and atomic layer deposition (“ALD”). Chemical vapor deposition processes, such as metalorganic chemical vapor deposition (“MOCVD”), deposit a metal layer by decomposing organometallic precursor compounds at elevated temperatures, i.e., above room temperature, either atmospheric pressure or at reduced pressures. A wide variety of metals may be deposited using such CVD or MOCVD processes.
For semiconductor and electronic device applications, these organometallic precursor compounds must be highly pure and be substantially free of detectable levels of both metallic impurities, such as silicon and zinc, as well as oxygenated impurities. Oxygenated impurities are typically present from the solvents used to prepare such organometallic compounds, and are also present from other adventitious sources of moisture or oxygen.
For certain applications where high speed and frequency response of an electronic device is desired, silicon-only devices, e.g. silicon bipolar transistors, perform marginally and the introduction of germanium is necessary to obtain the desired functionality. In a heterojunction bipolar transistor (“HBT”), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The silicon-germanium HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. The speed and frequency response of a silicon-germanium HBT are comparable to more expensive gallium-arsenide HBTs.
The higher gain, speeds, and frequency response of silicon-germanium HBTs have been achieved as a result of certain advantages of silicon-germanium not available with pure silicon, for example, narrower band gap and reduced resistivity. Silicon-germanium may be epitaxially grown on a silicon substrate using conventional silicon processing and tools. This technique allows one to engineer device properties such as the energy band structure and carrier mobility. For example, it is known in the art that grading the concentration of germanium in the silicon-germanium base builds into the HBT device an electric field or potential gradient, which accelerates the carriers across the base, thereby increasing the speed of the HBT device compared to a silicon-only device. A common method for fabricating silicon and silicon-germanium devices is by CVD. A reduced pressure chemical vapor deposition technique (“RPCVD”) used to fabricate the HBT device allows for a controlled grading of germanium concentration across the base layer as well as precise control over the doping profile.
Germane (GeH4) is the conventional precursor for germanium deposition. Germane is a gas under standard conditions and is difficult to handle. As germane is toxic, processes employing germane require extensive safety procedures and equipment. Germane typically requires film growth temperatures of approximately 500° C. for thermal CVD applications. Such decomposition temperatures are not always suitable, such as in applications where there is a need for lower temperatures, e.g. 200° C. Other CVD applications require higher growth temperatures, e.g. 700–1100° C., which cause germane to break up prematurely which, in turn, leads to the formation of particles and a reduction in metal film growth rates. A further problem with germanium precursors arises in silicon-germanium deposition when a relatively stable silicon precursor and a relatively unstable germanium precursor (germane) are used to deposit a silicon-germanium film, the differences in precursor stability makes control of the silicon-germanium composition difficult.
U.S. patent application Ser. No. 2003/0111013 (Oosterlaken et al.) discloses an apparatus for the deposition of silicon germanium layers. This application discloses certain source compounds for the vapor deposition of germanium, such as mono-, di- tri- and tetra-chlorogermanes. Such compounds may not be suitable for all germanium vapor deposition applications as their decomposition temperatures may be too low. For example, monochlorogermane is known to decompose at temperatures as low as 25° C.
There remains a need for germanium precursors that offer an optimized deposition of germanium-containing films at various growth temperatures. Such growth temperatures determine the properties of the germanium-containing film. A limitation in growth temperature limits the full exploitation of the capabilities of a germanium-containing film. There remains a need for germanium precursors for CVD that are safer to handle.